An optical transmitter includes an optical modulator having first and second waveguides to modulate carrier light at each of the waveguides using a driving signal with 2*n intensity levels (n is an integer 1 or greater); and a phase shifter to cause a phase difference between a first optical signal and a second optical signal output from the first waveguide and the second waveguide, respectively. A photodetector converts a portion of a multilevel optical modulation signal acquired by combining the first optical signal and the second optical signals into an electrical signal. A monitor detects a change in an alternating current component in the detected modulation signal. A controller controls at least one of a first bias voltage and a second bias voltage being supplied to the first waveguide and the second waveguide, respectively, so as to increase the power value of the alternating current component.
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March 30, 2012
April 21, 2015
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